It depends on the type of Mac you're upgrading. I have a late 2009 27" iMac that I "upgraded" prior to leaving for work this morning. The maddening this is that while I have no issues with my Bluetooth keyboard, I can only move the mouse cursor with my Magic Trackpad. When I checked System Preferences, the Bluetooth icon was simple not there! I saw the Bluetooth icon in the Menu Bar, but the message stated that Bluetooth was unavailable!
I don't know what's going on here, but I'm going to go out on a limb and say that integrated systems (ie., laptops only) are probably pretty safe with this "upgrade", but iMacs and Mac Pros could be a bit dicy. I'm not sure what to do at this point, but will probably attempt a re-install, and should that fail, go ahead and do a "nuke and pave." After all, it's only been six years since I made the switch - I still remember that option from my Windows days